It's probably the chute. That's kind of a big deal. If he just took the beer and stormed off the plane back up the boarding tunnel, even after chewing out the passengers, I don't think there'd be any legal repercussions pending. I don't see the reckless endangerment or trespassing at all under the circumstances. Maybe they tacked those off to bargain with later. I do think there should be some "destruction of property" charge because of the large cost associated with the chute and deplaning/replaning all the pax.
It’s all about the chute. The wreckless endangerment probably is because the plane can’t taxi with the chute deployed, meaning (1) the passengers and other crew had to deplane on the active tarmac (dangerous) and (2) the plane was now a stuck-in-place large obstacle for other planes (dangerous). That’s not even getting into sticking a big sail on the side of a full airplane.
The trespassing probably is for going onto the active part of the tarmac. Flight attendants generally aren’t cleared for that. That’s also potentially dangerous for others.
My bet is that he does some time, possibly federal, unless (or maybe even if) he cuts a very nice plea deal.